1. Field of the Invention
The invention relates to a control device for a four wheel drive (i.e. 4WD) vehicle that is provided with a disconnection mechanism for the connection/disconnection of a power transmission path for transmitting power of a driving force source to auxiliary driving wheels and right and left clutches disposed in respective power transmission paths to both of the right and left auxiliary driving wheels.
2. Description of Related Art
A 4WD vehicle is widely known that is provided with a disconnection mechanism for the disconnection of a power transmission path for transmitting a part of power of a driving force source transmitted to main driving wheels to auxiliary driving wheels, which become driven wheels during two wheel driving (i.e. 2WD), during 4 wheel drive (i.e. 4WD) and a first clutch and a second clutch disposed in respective power transmission paths to the right and left auxiliary driving wheels. One of the examples is the vehicle that is disclosed in Japanese Patent Application Publication No. 2010-100280 (JP 2010-100280A). JP 2010-100280A discloses a 4WD vehicle that is provided with a disconnection mechanism (dog clutch) on the front wheels side and is provided with two multi-plate clutches on the rear wheels side. According to JP 2010-100280A, the dog clutch and the two multi-plate clutches are released during 2WD so that the rotation of a torque transmission section (corresponding to a propeller shaft) or the like between the clutches (dog clutch and multi-plate clutches) is stopped for fuel economy improvement and the two multi-plate clutches can be separately controlled during 4WD when the clutches are engaged so that the differential function of a differential gear is realized by the two multi-plate clutches without any rear differential gear.
In the vehicle that is disclosed in JP 2010-100280A, the 2WD state and the 4WD state are alternatively switched. In general, switching to the 4WD state is carried out in the event of wheel slip, a strong acceleration request from a driver, or the like as disclosed in JP 2010-100280A. Switching to the 4WD state that is carried out to improve the straight traveling stability of the vehicle against a disturbance such as a road surface disturbance and crosswind is also considerable apart from this. However, more-than-necessary switching to the 4WD state for the improvement of the straight traveling stability of the vehicle despite an acceleration request which is not strong as in steady traveling (constant-speed traveling) is likely to result in fuel economy deterioration. Also, if the 2WD state is maintained for a long period of time for fuel economy improvement despite the occurrence of a disturbance, it may be impossible to promptly improve the straight traveling stability of the vehicle against the disturbance. The above-described problems are not known, and no technique for improving straight traveling stability against a disturbance and suppressing fuel economy deterioration at the same time has been proposed.